otography that I have obtained up to the present. this case the surface of the condenser is one uare foot, and the discharge is taken through inds of copper about two inches broad, and not more an about four inches long apiece. Extra good conct is made between these copper bands and the tinil surface by long radiating tongues of copper-foil ldered to the end of the copper bands. The knobs are latinum, but this seems no better than copper. The hole of the light is extinct in less than one-millionth f a second, while the first blaze, which is practically the hole spark, the tail being in comparison of no conseuence, does not last so long as a ten-millionth of a econd; in other words, it lasts so short a time that it ears the same relation to one second that one second ears to four months; or again, a magazine rifle bullet, ravelling at the enormous speed that is now attained by he use of this weapon, cannot go more than one fourhundredth of an inch in this time. Other sparks of still less duration were examined, but this was chosen for the purpose of photographing bullets.' Now, having obtained a suitable flash of light, I must next show how a spark may be used for the purpose of photographing a bullet in its passage This was first done by Prof. E. Mach, of Prague, whose method is illustrated by the diagram Fig. 3. The squares on the FIG. 3. steamer. In fact, the whole atmospheric phenomenon accompanying the bullet is not unlike that seen on the surface of water surrounding and behind a steamship. These were seen for the first time, and their visibility by this method was, I believe, predicted by Prof. Mach before he made his first experiment. The part that I have played in this matter is after all very subordinate. I have attempted to simplify the means, and the results which may be obtained by the modified method which I have devised, are, I believe, in some respects-I don't say in all-clearer and more instructive than those obtained by the more elaborate device of Prof. Mach. a E Fig. 4 is a diagram of the apparatus that I have used. C is a plate of windowglass with a square foot of tin-foil on either side. This condenser is charged until its potential is not sufficient to make a spark at each of the gaps E and E', though it would, if either of these were made to conduct, immediately cause spark to form at the other. c is a Leyden jar of very small capacity connected with C by wire, as shown by the continuous lines, and by string wetted with a solution of chloride of calcium, as shown by the dotted line. So long as the gap at B is open this little condenser, which is kept at the same potential as the large condenser by means of the wire and wet string, is similarly unable to make sparks both at B and E', but it could, if B were closed, at once discharge at E'. right-hand side represent certain electrical apparatus by Now suppose the bullet means of which a Leyden jar (J) is charged with elec- to join the wires at B, a tricity to such an extent that, while it is unable to make minute spark is made at two sparks at B and A, it is nevertheless able to, B and at E' by the disand at once will, make a spark at B when the second charge of c, immediately gap at A is closed by a bullet or other conductor. C, finding one of its gaps The dotted lines represent wires through which the dis- E' in a conducting state, charge then takes place. The spark at B, magnified discharges at E, making by the lens in front of it, then fills the field lens La brilliant spark, which with light, so that the camera K focussed upon the casts a shadow of the spark gap A will then receive an image of the bullet as it bullet, &c.. upon passes, and thus a photograph is secured. I am able to photographic plate P. Though this is simple enough, show two of these which Prof. Mach has kindly for- the ends that are gained by this contrivance are not so warded to me, and what I wish to point out is that in obvious. In the first place the discharge circuit of C, each of these photographs-and this is perhaps the most via E and E' is made of very short broad bands of interesting feature which any of these exhibit-there are copper, a form which favours both the brilliancy and the seen, besides the bullet and the wires which the bullet shortness of duration of the sparks; further, the double strikes in its journey, certain curious shades, one in ad- gap, of which E' may be the longer, causes the intensity vance of the bullet and one from the tail, while a trail is of the light of either spark to be greater than it would be left behind very like that seen in the wake of a screw if the other one did not exist in a particular case the light of the shorter was increased six or eightfold-at the same time the duration is not greatly affected. For this reason shadow is almost as sharp as if the light came from a the spark at E may be made very short, so that the point. The spark formed at B, which is due to the discharge of c only, is very feeble, so that it is unable to act on the plate, whereas, had the discharge of C been carried round by B, the light at this point would hopelessly have spoilt the plate, and at the same time the light at E would have been feebler and would have lasted longer. These sparks were made to go off at the time that the mirror was facing towards the photographic plate by the employment of the same device as that described below in connection with Fig. 4. Ou the axle of the mirror an insulated tail of aluminium was secured, so as nearly to bridge a gap in the discharge circuit of an auxiliary jar of small capacity, there being a gap common to both circuits. A self-induction coil was used instead of the wet string, as being for this purpose preferable. The length of time that the spark lasted was thus measured without taking the electricity round by the mirror, which would have been quite sufficient to modify the duration of the discharge, and it was easier than making and adjusting a second reflecting mirror, which would have answered the same purpose. See NATURE, VOL. xlii. p. 250. the FIG. 4. B The wet string, while it is for the purpose of keeping the condenser c charged a perfect conductor, is nevertheless, when this discharges at E' and B, practically a perfect insulator; if it were replaced by wire then C would also wholly or partially discharge itself by B and E'. Finally, in avoiding all lenses one is free from the considerable absorption of the more refrangible rays which sparks provide in great abundance, and which are largely absorbed by glass. On the other hand the photograph is a mere shadow, but this is no drawback, for the bullet itself is on either system a mere silhouette, whereas the atmospheric phenomena are more sharply defined, and their character is more clearly indicated without lenses than is possible when they are employed. Fig. 5 is a photograph of the apparatus set up in one of the passages in the Royal College of Science, in which the experiments were made. It is apparently of the rudest possible construction. The rifle seen on the left of the figure is of course made to rest freely on six points, in order that its position every time it is fired may through these holes is not diffused in any harmful mare The large box at the back is a case 5 ft. long, filled bran which stops the bullets gently without marking ther The little condenser is just below the rectangular longation of the photographic box, the large condense the vertical square sheet seen just to the right. electrical machine used to charge the condensers is s on the table. It is a very beautiful 12-plate Wimshe machine made by Mr. Wimshurst and presented to: Physical Laboratory. This machine not only works certainty but is so regular in its working that electrometric apparatus is necessary. All that has to done is to count the number of turns of the handle wh are required to produce the sparks at E and E' when gap at B is not joined, and to count the number w are sufficient to produce a spark at E when the gap r is suddenly closed. Then if the rifle is fired after number of turns between these, but by preference nee the larger than the smaller number, the potential w right, the spark E, inside the box, and the spark E', t be the same. FIG. 5. The bullet then traverses precisely the same course, so that wires placed in the line between holes in two cards made by one shot will be hit by the next. The two wires which the bullet joins as it passes by are set up in the box seen in the middle of the figure with the lid propped up so as to show the interior. The photographic plate is on the left-hand side and the spark when made is just within the rectangular prolongation on the righthand side. Paper tubes with paper ends are placed on each side of the box to allow the bullet to enter and leave, and yet not permit any daylight to fall directly on the plate. All is black inside, and so the small amount of light which does enter the box 1 Six independent points of support are required for a geometrical clamp. In this case a V support near the muzzle supplied two, a V support near the breach two more points, the rifle was pressed forward until a projection under the muzzle rested against the front V, thus allowing freedom of recoil, but otherwise preventing all uncertainty of position except that due to rotation in the V's which is made impossible by the sixth point, that is, the lower end of the stock resting sideways against a leather covered wooden bracket fastened to the same table to which the V's were attached. is in sight outside the box, will be let off, and plate is exposed a photograph will be taken. If by the E' spark is not seen then there is no occas waste the plate, another bullet may be fired resetting the wires and the result will be as good as shot bad not failed. When all is in order a failure of kind is very rare. I also arranged a tube in the s the box with a pocket telescope fixed in it and foc on the wires. If a piece of white card or paper is p in the line of vision and so as to be illuminated spark let off as above described but preferably nearer the card, the bullet will be seen by any one lo through the telescope. I took this down, howeve once, as the photograph showed more than co be seen by the eye. The box seen just to the righ rifle with a coil of wire upon it is the one in whic revolving mirror was fixed, and in which the tra sparks made near the door at the end of the pas were photographed. The apparatus for photogra he bullets was put together and set up by Mr. Barton, a student, whose very skilful help in the matter and after was put together. It was taken to see if the idea would practically succeed, merely using for the purpose bits of wire and other things to be found in any laboratory, which were set up in a dark room in less than an hour. The first shot was successful, but the sharpness of the photograph is not what it might be, owing to the fact that I used, for the sake of the brilliant light, a spark taken between magnesium terminals. However, the bullet is clearly enough defined, as are the wires which it has just struck. This is a photograph of a pistol bullet travelling only 750 feet a second. You will notice that unlike that taken by Prof. Mach, which represented a shot going at a much higher speed, this photograph shows no atmospheric phenomena surrounding the bullet. I would only add, in connection with this photograph, that by some accident the wad remained attached to the bullet in this case forming the enlarged tail. I do not know if this often happens; it must, if it does, seriously disturb the flight of the projectile, and introduce an anomaly that might not easily be accounted FIG. 7. The first photograph which I am able to show was appears perfectly sharp. There is no sign of any moveen at Christmas, before the apparatus just described ment whatever in so far as the bullet itself is concerned. But now that we are dealing with a higher speed, namely, 1295 feet a second, there is evidence of the movement of the bullet in the form of a wave of compressed air in front and of other waves at the side of and behind the bullet. I shall explain this in a moment, but I would rather first show another photograph, Fig. 7, of a bullet travelling at a still higher speed, a magazine rifle bullet travelling about 2000 feet a second, in which these air waves are still more conspicuous, and in which a glance is sufficient to make it evident that the waves are much more inclined to the vertical than in the previous case. Now as it may not be evident why these waves of air are formed, why their inclination varies with the speed, or why existing they are visible at all, a short explanation may not be out of place, more especially as they form the most interesting feature in the remaining photographs that I have to exhibit, which cannot, as a matter of fact, be properly interpreted without frequent reference to them. I would first ask you to examine some still water into which a needle held vertically is allowed to dip. If you move the needle very slowly not a ripple is formed on the surface of the water; but as the needle is moved more quickly at first a speed is reached at which feeble waves appear, and then as the speed increases a swallowtail pattern appears, the angle between the two tails become less as the velocity gets higher. Now in the case of water-waves the velocity with which they travel depends on the distance between one and the next, and for a reason into which I must not now enter either very long or very short waves travel more quickly than waves of moderate dimensions. If they are about two-thirds of an inch long they travel most slowly-about 9 inches a second. Now so long as the needle is travelling less quickly than this no disturbance is made; but when this speed is exceeded the swallow-tail appears. Suppose, for example, the velocity of the needle to be double the minimum wave velocity for water, i.e. let the needle move at 18 inches a second, and let it at any moment have arrived at the point, Fig. 8. Then any disturbance, started will be smaller, and the angle between Lp and Mp will be come less, while when the velocity is made less the revere happens, until at last Aa Bb, &c. = Ap Bp, &c., and the when they exceed these quantities no lines Lp Mp be drawn touching all these circles, there is no wr surface which the disturbances from all the success. points can conspire to produce, and the consequence there is still water. Now consider the case of a bullet moving through air. Here again we are dealing with a case in whic.. wave cannot travel at less than a certain speed which obviously the velocity of sound (1100 feet a second ne ordinary circumstances), but, as in the case of surface waves on water, higher speeds are possible when the ware is one of very great intensity. The conditions in the cases are therefore very nearly parallel; if the bule: travelling at less than the minimum speed no e should be formed-the pistol bullet at 750 feet a secur did not show any—if the bullet is travelling at higher spee than 1100 feet a second waves should be formed wh should include a sharper angle as the speed is made increase. This was found to be so in the case of th Martini-Henry and the magazine rifle bullet. The curved form of the wave near the apex is due to the fact that when it is very intense, when the compression. very great, the velocity of travel is greater and, immediate in front of the bullet, the air is compressed to so great. exent that the wave at this part can travel at the spee. the bullet itself. The reason why the waves should be visible at all not difficult to follow. Consider a shell of compresse. air though which rays of light from a point are made traverse. These rays travel in straight lines, except whe they meet a medium of different density, and the des this is and the more nearly they meet this at a grain incidence the more they will be bent towards the perpend cular. In comparison with water or glass a layer of co pressed air has very little refractive power, and so ras which strike the shell anywhere except at the extreme edge are practically uninfluenced in their course strike the plate practically in the same place that would do if the shell of compressed air had not be FIG. 8. when it was at the point A, must have travelled as far as the circle aaa in which Aa is half Ap, similarly for any number of points BC, &c., between A and any disturbance must have travelled as far as the corresponding circles bb, cc, &c., the result is that along a pair of lines, PL, PM, touching all the circles that could be drawn in this way, a wave will be found, and it is clear that as the velocity of the point is made greater the successive radii Aa Bb, &c., will become in proportion to Apless, the circles Compressed air gives rise to an image on the plate, in which there is a dark line and a light line within it. Similarly a wave of rarefaction must produce a light ine with a dark one within it. An examination of the photograph Fig. 7 will make it evident that not only is the nead wave a wave of compression, but the wave, which starts from the end of a kind of vena contracta behind the bullet, is also a wave of compression. It is a curious fact which requires explanation that the head and tail waves are not parallel to one another, and they do not show any sign that they would become parallel if they were continued indefinitely. This can only be due to either the tail of the bullet travelling considerably faster than the head, or to the actual velocity of propagation of the tail wave being less than that of the head wave. The effect observed is true and is not optical, being neither due to the refractive effect of the outer shell disturbing rays which are tangential to the inner shell, nor to an effect of perspective, for though the projection of a cone from a point upon a plane is only seen of the proper angle, when the perpendicular, dropped from the point upon the plane, passes through the vertex of the cone, yet when, as in the case of Fig. 11, where it passes within both cones, and more within the outer one than the inner one, the effect is to make the projections of both of a greater obtuseness, and of the outer one to a greater extent than the inner one; nevertheless an examination of the amount of this effect of perspective made by Mr. Barton showed that the distortion was not sufficient to be noticeable, as the difference in the acuteness of the cones certainly is. (To be continued.) NOTES. ADMISSION to the Croonian Lecture, which Prof. Virchow, as we have already announced, is to deliver before the Royal Society at 4.30 p.m. on the 16th inst., will be by ticket, which may be obtained from the assistant secretary by introduction of a Fellow of the Society. THERE will be widespread regret at the announcement which we now make that the distinguished geologist, Prof. K. A. Lossen, of Berlin, died there on the 24th ult. He had been ailing for some time, and suffered severely before he entered anto his rest. In spite of the deafness which necessarily restricted his intercourse with men of science, he had formed a wide circle of friends who learned to appreciate the simplicity, candour, and geniality of his character, while at the same time they came to respect and admire more and more his wide range of knowledge, and that marvellous and apparently intuitive perception of the true characters of rocks which made him probably the best field-petrographer in Germany. We have received news of the death of Cav. Giuseppe Antonio Pasquale, for many years professor of botany in the University of Naples, and director of the botanic garden. Prof. Pasquale was the author of numerous articles on botany and cognate subjects. His earliest works of which we have cognisance were on the flora of Capri (1840), and the flora of Vesuvius (1842). In 1869 he published a more complete "Flora Vesuviana, confronte con quella dell' isola di Capri." He appears to have been appointed to the post of director of the Naples Botanic Garden in 1866, and the following year he It may be worth while to point out that the dark and light lines are, and ought to be, parallel to one another as soon as they are so far away from the shadow of the bullet as to be practically straight lines. For if the thickness of the shell is divided up into a series of elements the ray passing through any one of these will meet with a refractive medium, which is less effective as the diameter of the part of the shell considered is greater, while the refractive angles of the elementary prisms become inclined more so as to compensate for the diminished density. published a catalogue of the plants cultivated there, together with a brief history of the garden. THE German Government has established a biological Institute on the island of Heligoland, and has appointed Dr. Kuckuck its botanical director. PROF. SCHWEINFURTH landed at Port Said on January 7, for an expedition into Upper Egypt which is intended to extend over several months. Dr. D. Riva, who accompanied Schweinfurth on his last journey, has undertaken an expedition to Eastern Africa in the vicinity of the river Giuba. herbarium of Dr. Gottsche have passed into the possession of THE moss herbarium of Dr. Rehmann and the Hepaticathe Botanical Museum of Berlin; the Botanical Museum of the University of Vienna has acquired the moss-herbarium of Hoppe; and the Botanical Institute of the German University at Prague the greater part of the valuable library of Prof. Willkomm. THE Reale Istituto Veneto di Scienze, lettere ed arti proposes the following prize subjects:-(1) A lithological, mineralogical, and chemical investigation of the rocky, sandy, earthy, and saline materials brought down under various conditions by one of the chief rivers of Venetia from the Alpine valleys, and deposited at various distances from the base of the Alps to the sea (prize, 3000 lire, date December 31, 1893). (2) A compendium of the history of mathematics, with a mathematical chrestomathy containing extracts from mathematical works of antiquity, the middle ages, the renaissance, and recent times down to Gauss (indicating in each case the reason for introducing the extracts), (prize and date the same). Papers may be written in Italian, Latin, French, German, or English, and are to be sent in to the secretary with motto and sealed packet. SIR ANDREW BARCLAY WALKER, who died on Monday, did much to promote intellectual life in Liverpool. The University College of that city has good reason to remember him as one of its most generous benefactors. He assumed the entire pecuniary responsibility for the erection of the Walker engineering laboratories, which cost about £20,000. MR. O. M. EDWARDS, who was appointed to investigate the various conditions which have to be taken into account in connection with the proposal for the establishment of a Welsh University, has completed his inquiries and forwarded his report to the Vice-President of the Committee of Council on Education. A writer in the University Correspondent says the report is practically a pamphlet of about eighteen octavo pages, containing a short account of the origin and progress of the the information already possessed by the Department of Educaeducational movement in Wales, and intended to supplement tion on this head. It contains a succinct epitome of the various schemes proposed-the Shrewsbury Charter, the proposals of Dr. Roberts and Prof. Evans; gives the state of efficiency of Lampeter and the three Welsh colleges; contrasts them with those at Leeds and Manchester; and points out how far, more or less, the Welsh institutions are prepared and adapted, in point of staff, students, accommodation, and appliances, to receive similar powers. THE Municipal Council of Paris has been giving names to some new streets, and changing those by which various old streets have hitherto been known. The names selected for use are for the most part those of illustrious Frenchmen, and it is significant that among them are some well-known men of science. The Rue du Battoir, for instance, is henceforth to be called the Rue Quatrefages, in memory of the famous anthropologist; and the Rue Claude-Velle faux becomes the Rue Charles-Robin, in memory of the great physician. A new street is called after |